Nitrosomonas and Nitrobacter were once thought to be the principal nitrifying bacteria in freshwater aquaria. However, more recently it has been suggested that Nitrospira and Nitrosospira spp. actually make up the bulk of the nitrifying biofilm.
Nitrospira and Nitrosospira bacteria are available for sale as a live culture, in a product called "Bio Spira". I've never used it myself, but there have been quite a few positive reviews for it on this very forum.